Source for evaporation in a vacuum



' G. L. GALLET SOURCE FOR EVAPORATION IN A VACUUM Ndir. 17,1970

2 Sheets-Sheet 1 Filed Oct. 31, 1968 "Nov. 17;19'10' qgg-Aujgf "354L315" SOURCE FOR EVAPORATION IN A VACUUM Filed Oct. 31, 1968 v 2 Sheets-Sheet 2 United States Patent US. Cl. 219271 6 Claims ABSTRACT OF THE DISCLOSURE A high output vapor source for use in vacuum aluminizing. An aluminum wire is continuously drawn over the plane, horizontal surface of a body which is composed of boron nitride which is traversed by a heating resistor for melting the Wire. Fusion takes place as the metal is melted into a thin liquid coating and spread over a large area where immediate vaporization occurs.

The present invention relates to a vapor source for metalization in a vacuum and more particularly for alumimzlng.

The technical difliculties encountered in providing an appropriate source for the evaporation of certain metals are well known, more particularly in the case of metals such as aluminum which are endowed with a high chemical activity in the neighborhood of their evaporation temperature. Aluminum reduces substantially all metallic oxides, permits alloying with most metals, and in its normal state, is covered on its surface with an oxide film. All these characteristics make the evaporation of aluminum particularly difficult to accomplish. Its high chemical affinity and its aptitude to diffuse preclude the use of the usual crucibles which are capable of containing liquid metal. A novel solution therefore consists in utilizing the quasi-instantaneous vaporization of an aluminum filament continuously brought into the neighborhood of a body at very high temperature. In this way, the speed of travel of the filament permits ready and flexible control of the rate of vapor supply. However, difiiculties remain, especially if a considerable rate of vapor supply is to be obtained. More particularly, it is diificult to provide a heating element which is not corroded by evaporated aluminum.

In addition, it is known to construct the vapor source in two separate parts, one of which the evaporator, receives the metal to be vaporized and the other, the heat generator, is disposed in immediate proximity to the evaporator so as to supply thereto by radiation the caloric power necessary for maintaining the appropriate temperature for vaporization. It is then easier to construct the evaporator of a material which resists liquid aluminum, since it is not necessary for this material to have and to retain electrical characteristics which enable it to be electrically heated. Likewise, it is no longer necessary for the material, of which the heat generator consists, to be resistant to liquid aluminum.

It has also been proposed, that the heat generator, consisting of a rod-shaped electrical resistor, should be completely surrounded by the evaporator so that the rod is shielded from the aluminum vapors. Unfortunately, serious problems arise at the inlet and outlet joints of the electrical connections by which the generator is supplied.

These joints must undergo very considerable temperature variations when the operation of the vapor source is started and stopped, and they are subjected during operation to the constant presence of aluminum vapor. This results in solid or liquid aluminum deposits Which cause 3,541,301 Patented Nov. 17, 1970 cooling of the evaporator and a chemical attack on the electrical connections or the rod.

It has been observed in this connection that when the surface of such a rod is too close to another surface, a drop of liquid aluminum may remain between its two surfaces despite the high temperature of the rod, which is above the vaporization temperature of the aluminum because surface tension affects (capillarity) may prevent the vaporization.

The present invention has for its object, the construction of a source of metallic vapor for metalization under a rarefied atmosphere, which source provides a high rate of vapor supply, while avoiding to the largest extent possible the powerful action of the metal deposits.

The present invention also concerns a source of this type which is geometrically well defined, that is, such that the origin of the rectilinear paths of the vapor molecules are directed towards the surfaces to be metalized is limited to a predetermined appropriate surface.

The vapor source according to the invention comprises, in addition, to an evaporator and a heat generator is indicated in the foregoing, a metallic reflector which is appropriately cooled so that the vaporized metal cannot remain liquid in contact therewith. The reflector must be made of a metal which is a good heat conductor. One of its functions is to return to the evaporator the heat radiation which has emanated therefrom, so as to avoid cooling of the evaporator. Another function of the reflector is to provide a basic structure for supporting the evaporator in the heat generator.

Some advantages of this metallic vapor source are, that it is capable of continuous operation by quasi-instantaneous evaporation over a large surface and it is also capable of supplying vapor at a high rate of flow, for example, 48 grams of aluminum per hour per source point with the lifetime readily reaching tens of hours. Also, the rate of fiow of vapor is readily controllable by controlling the unwinding speed of the reel. Other advantages are the preservation of the heating element from all contamination due to splashing with liquid metal, the cleanliness of the deposits obtained, the possibility of evaporating an alloy without changing the proportion of the constituents, and the possibility of obtaining deposits of mixed layers by simultaneous and controlled evaporation from two or more sources.

The preferred embodiment of the metallic vapor source according to the present invention, will be described as follows with reference to the accompanying figures.

FIG. 1 is an exploded view in perspective of the source of metallic vapor according to the invention.

FIGS. 2 and 3 illustrate respectively transverse and longitudinal sections to the same vapor source provided with its reflector.

The source is intended to supply aluminum vapor at a high rate of flow, that is, 300 mm. /min. or 0.8 g./min. for each supply filament, it is understood, however, that a number of filaments may be simultaneously evaporated at the same source. Such a rate of flow requires a vapor pressure of the aluminum higher than 0.1 mm. Hg at a corresponding temperature above 1400 C., and necessitates a change of the source with conventional technical means after operation for one to two hours. Such changing of the source is obviated if the source of the present invention is employed.

As can be seen in the figures, an evaporator 1 consists of a refractory body, the external shape of which is that of a prism of rectangular cross-section having a width of about 15 mm., a length of about 50 mm., and a height of about 15 mm. The refractory body which must be resistant to liquid aluminum, advantageously consists of boron nitride, but it is obvious that other refractory materials could be employed, for example, high density graphite.

The refractory body has a horizontal rectangular upper face. Its lower face is formed with a downwardly opening groove. Disposed in the groove is a heating resistor 3 in the form of an elongated rod, which consists of a refractory material such as titanium boride, but which could consist of other materials such as, for example, graphite. The material selected must be resistant to alumina vapor but not necessarily to liquid aluminum.

The elongated rod or resistor 3 is disposed inside the groove without coming in contact with it at any point. The minimum clearance between the rod and the groove is approximately 1.5 mm. It may be desirable to raise this value to 2 mm. but in any case, it must not be less than 1 mm. It does not appear to be advantageous to increase the clearance beyond 3 mm.

The resistor 3 is brought to a temperature higher than that which it is desirable to have on the upper face of the evaporator 1. The temperature on the upper face may be in the neighborhood of 1400 C. and that of the resistor about 1700 C. The end of the aluminum filament 2 has a diameter of 0.5 mm. which is maintained in contact with the upper face of the evaporator 1. For this purpose, the filament 2 is continuously unwound by means of driving rollers from a reel 5, at a speed of between cm. and 2 m. per minute. The filament 2 is guided by means set forth in British patent application No. l7,384/ 68 of Oct. 4, 1968.

On impact with the refractory body 1, the filament 2 melts and forms a thin liquid film in a zone 4, which is maintained relatively cold by the continuous supply of the heat of fusion, and in which the liquid wets the body 1. The aluminum travels from zone 4 towards the hotter zones as indicated by the arrows 8. This results from the fact that the temperature gradient of the refractory body 1 causes a distribution of the values of the surface tension of the liquid metal which results in a decrease in these values as the distance from the point of impact of the liquid metal increases. On coming in contact with these outer zones, the metal instantaneously evaporates over the whole peripheral zone, such as that bounded by the contour 7.

The propagation of the liquid aluminum preferably takes place along the length of the evaporator 1 parallel .to the generatrices of the prism. This is because a considerable thermal gradient exists on the surface of the evaporator 1. The elongate zone situated immediately above the resistor 3 is hotter by reason of its greater proximity to the resistor and by reason of its smaller thickness, which is of the order of 1 mm. It is smaller than in other regions of the evaporator 1, because of the plane form of the upper face of the evaporator 1 and also because of the fact that the base of the groove in the lower face of the evaporator is in the form of a cylinder having a semicircular cross-section.

The resistor 3 has a circular cross-section and is disposed coaxially at the base of the groove. The thickness is advantageously, but not necessarily, further decreased above the resistor 3- by the formation of a recess in the upper face of the evaporator 1. The recess has a length of about 35 mm. in the described example, and is bounded by a sharp edge. The recess has as its object the limiting of the active surface of the aluminum vapor source, because in the aluminizing operations for which the source is intended, it is generally desirable to bound distinctly the metalized zones from the zones which are not to be metalized. This may be done with the aid of masks, appropriately disposed to intercept certain rectilinear paths of vapor molecules. This delimitation is more distinctly defined in proportion as the source is smaller.

The function of the recess is not to constitute a reserve of liquid aluminum, because such a reserve would cause fluctuations in the rate of vapor flow by reason of the unavoidable existance of a very thin and unstable layer of alumina on the surface of the liquid aluminum. Moreover, the presence of such a reserve would cause variations in the composition of the vapor in the case of the evaporation of an alloy.

The formation of such a reserve is avoided by choosing a sufliciently high temperature on the surface of the evaporator 1 and a sufliciently low rate of supply of the aluminum filament 2, in order that the later may vaporize almost instantaneously when it comes into contact with the evaporator. There is disposed around the evaporator 1 a reflector 11 open at the top, which consists of solid copper cooled by fluid moving in the direction of arrows 22 through the U-shaped tube 23 embedded therein. The reflector 11 has the object of minimizing the loss of heat which occurs due to radiation in the vacuum. The evaporator 1 is supported above the space of the reflector 11 by four dogs 12, which consist of cooled copper and are secured to the reflector 11. The dogs 12 engage apertures 13 situated at the bottom of the evaporator 1 close to their ends. This arrangement facilitates thermal contact between the evaporator 1 and the reflector 11 to a greater extent because the apertures 13 are further from the active part of the evaporator 1.

In order to increase this spacing and in order to avoid excessive proximity of the ends of the upper face of the evaporator 1 to the outside walls of the reflector 11, the outer faces of the evaporator 1 are not vertical, but are inclined in such a direction that the lower part of the evaporator has a greater length than its upper part. Similarly, the walls of the reflector 11 are so inclined that the surface opened at the top of the reflector 11 is larger than that of its base.

These configurations make it possible to prevent aluminum condensation from creating undesirable thermal contacts between the reflector 11 and the evaporator 1.

It might be felt that sufiicient spacing is more important between the reflector 11 and the evaporator 1 than between the evaporator 11 and the resistor 3. The resistor 3 is brought to a temperature very much higher than the evaporation temperature of the aluminum and thus, it would seem that a drop of liquid aluminum. could not remain in contact with it. In practice, however, if the spacing is too great, a drop of liquid would remain in contact with the resistor 3 because the pressure in the drop, due to its surface tension, may prevent evaporation thereof.

The resistor '3 is supplied with an electric current and supported at its ends by a push member 20 and an abutment member 21, both of which consist of cooled copper and extend through the end walls of the reflector 11. The push member 20 is provided with a spring which provides the thrust appropriate for supporting the resistor 3. The spring is not shown, since it is situated at a distance from the source proper. The faces of the push member and of the abutment 21 are slightly hollowed so as to form a seating for the ends of the resistor 3-.

In order that the resistor 3 may not be electrically short-circuited by the metallic reflector 11, appropriate insulating means (not shown) are provided. These means may consist in a separation of said reflector into two parts. Of course, if necessary, a number of filaments such as two may be simultaneously evaporated at various points of the upper face of the body 1.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What I claim is:

1. A metallic vapor source for metallization in a rarefied atmosphere, of the type without liquid bath, and with indirect heating, which source comprises:

(a) an elongated evaporator consisting of a body of refractory material having a transverse cross section in the shape of an inverted U with a horizontal, flat upper surface,

(b) means for continuously supplying to said upper face the metal to be vaporized,

(c) a heat generator consisting of an electrical resistor in the form of a horizontal rod disposed in said -U- shaped portion and spaced from said evaporator for heating said upper surface, the ends of this rod being located beyond the ends of said evaporator,

(d) an upwardly opening metallic reflector surrounding said evaporator and said heat generator,

(e) means for cooling said reflector,

(f) means associated with said reflector for supporting said evaporator and said heat generator relative to each other and to said reflector such that the distances between the surfaces of said reflectors, said evaporator and said heat generator are always sufficient to prevent a drop of liquid metal from being held between said surfaces by capillary action.

2. A metallic vapor source according to claim 1 further comprising means for supplying power to said heat generator of sufiicient magnitude such that the metal supply to the faces instantaneously vaporize upon coming in contact with said face to preclude the formation of an appreciable reserve of metals thereon, in order to control the stream of metal vapor by varying the speed of incoming metallic wire.

3. The metallic vapor source according to claim 1 wherein the means for supporting said evaporator and said heat generator are dogs extending substantially horizontally from the side walls of said reflector towards the inside of the reflector, the evaporator being mounted on the free ends of said dogs.

4. A metallic vapor source according to claim 1 wherein the evaporator is of a general elongated and prismatic form, and wherein said U-shaped portion extends rectilinearly and horizontally from one end of the lower part of said evaporator to the other.

5. The vapor source according to claim 4 characterized in that the side faces of the evaporator are inclined in a direction such as to provide the lower face of the evaporator with a larger area than the area of its upper face.

6. The vapor source according to claim 1 wherein the upper face of the evaporator contains a recess of very small depth having steep edges which limit the surface over which the liquid metal spreads.

References Cited UNITED STATES PATENTS 2,664,853 1/ 1954 Schuler 118-49 3,036,549 5/1962 Iwata et a1. 219-426 X FOREIGN PATENTS 900,205 7/1962 Great Britain.

JOSEPH V. TRUHE, Primary Examiner P, W. GOWDEY, Assistant Examiner U.S. Cl. X.R. 118-48 

